Project Summary/Abstract Meiosis is a tightly regulated process that ensures formation of haploid gametes. Failure to segregate homologous chromosomes during meiosis results in aneuploidy, leading to chromosomal disorders such as Down syndrome and miscarriage. Incidences of homolog nondisjunction increase with oocyte age. A hypothesized cause of age-related nondisjunction is that aging oocytes are unable to maintain chiasmata, physical linkages between homologs, until they are ovulated. Chiasmata are formed via crossovers, genetic exchanges between homologs that are formed by repairing double-strand DNA breaks via homologous recombination. To ensure proper homolog segregation, the number and spatial patterning of crossovers is tightly regulated in a phenomenon known as “crossover patterning.” Understanding regulation of crossover formation and patterning, and therefore homologous recombination mechanism, is integral to combatting age-related infertility. Pathway choices within homologous recombination are traceable in products via heteroduplex DNA (hDNA), DNA in which the strands come from different parental chromosomes. The classic meiotic HR model indicates that a crossover is formed via a double Holliday junction (dHJ), a structure in which two DNA molecules are linked via criss-crossing of their strands at two adjacent sites. In this classic model, ligated dHJs give rise to all crossovers by being cleaved in one of two patterns, generating two possible hDNA signatures. The model predicts that both patterns are equally likely, yet only one of the hDNA signatures has been observed. This hDNA signature bias demands revision of the meiotic recombination model. Our lab has mapped hDNA at recombinants of a test locus in Drosophila melanogaster, but redefining the meiotic recombination model requires much more extensive analysis of hDNA than is possible with this methodology. To overcome this obstacle, I will pioneer “hetSeq”, a whole-genome sequencing technique to detect hDNA from meiotic products, to continue redefining this model. A further gap in our understanding of crossover regulation is that although crossover patterning has been observed since the early 1900s, its relationship to homologous recombination mechanism remains unclear. Many meiotic proteins have a known function in homologous recombination, and their depletion leads to crossover patterning defects. I am developing a mathematical model of recombination to test hypotheses about these proteins. To do this, I will alter aspects of crossover patterning within the model and compare the output to previously obtained experimental data from mutants lacking these proteins. I am additionally using this model to develop a simulation of recombination using whole- genome sequencing data. The proposed experiments will strengthen understanding of crossover regulation to provide guidance in combatting age-related infertility and aneuploidy.

Project Narrative Failures in homologous chromosome segregation during meiosis I is the leading cause of chromosomal disorders such as Down syndrome and miscarriages due to spontaneous abortion. Incidences of nondisjunction increase with maternal age due to inability of oocytes to maintain chiasmata, physical linkages between homologous chromosomes, until ovulation. This project will elucidate features of meiotic recombination and crossover patterning, the processes by which chiasmata are formed, giving new insights into and potential means to prevent age- related aneuploidy.